No splicing will be necessary. The mechanic will simply unplug and discard what is left of the exterior mirror harness from the door panel wiring harness. He will then plug the harness for the replacement mirror into the door panel wiring harness.
Here is a photo from eBay of a used right side mirror for a 1998-2005 GS. The cable from the mirror attaches to the wiring harness behind the door panel.